Process for electroplating a sintered ferrous article



Patented Dec. 20,, 1949 ST ES arswr OFFEC York No-Drawing; Appli'cationDecember 6,1945, Serial No. 633,211

3 Claims. (Cl. 204-347 This-inventionrelates: to a method of preparing. sintenad metal articles for electroplating. Specifically this: inventionrelates to: amethod which comprises (1) subjecting a sintered metal article to the action of: a basic solution, (2) subjecting the resulting article" to the action of an acid cleaning solution and 3) electrolytically deposit-- ing a metallic coating on the surface of the cleaned article using' an acid electroplating. bath. Morespecifically thisinvention relates to: a method of preparing a sintered metal articlefor electroplating which comprises subjecting said article; e. g;, a sintered ferrous article, to the action of a basic solution, and thereafter to the action of an acid cleaning solution prior to cleartrolyti'cally depositing a meta11iccoating;-e'.-g., a chromium coating, onthe surface ofthe sintere'd metal article.

An object of my invention isto prepare a sintered metal article for the step of electroplating the surface of said article with a metallic coating so" as toprevent a so-called pitted or sinking" effect-of themetallic coating deposited by the? plating process on the surface of the sin-- tered metal article. A further object is to prevent, in someca'ses, deterioration of thedeposited metallic surface on exposure to the air, e. g., rusting, caused by improper treatment of the sintered metal object prior to the electroplating operation.

In many cases it hasbeen desirable'to coat the slntered metal article with another, perhaps harder and smoother, metallic coating. This metallic: coating is, usually deposited onthe surfaceofthesinteredmetal article by an electrolytic plating process. However, great difiiculty has been ex 'aerienced in obtaining smooth, permanent finishes of these electroplated metallic coatings. The" common procedure in preparing the sintered article for the step of electroplating in an acid solution has been to immerse the sintered. article in a cleaning solution, which is usually a strongly acidic bath (also referred to as a.pickling solution), and thereafter to place the article in the. acid electroplating bath for deposition of the de .4

sired metallic coat on the. surface of the sintered metal article.

when the above procedure is followed, examination of the surface of the plated sintered metal article reveals that the article is unequally plated. In addition, a sinking efiect is noted; i. e.,, the deposited coatin appears to have sunk into the tiny pores found on the surface of sinteredmetal articles. This sinking efiect results in undesirably rough finishes on the sintered metal article comprises. a ferrous material, any breaks in the surface of the metallic coating, asa. re-- sult. of this sinking. effect, will. cause rusting of. the article. on exposure to the oxidizing effects. of: the air.-. Metallic articles prepared by other. methodssuchas, for examplaby casting of molten metal. or. by machining from unsinteredmetal blanks,. do not exhibit, the degree of porosity; shown by sinteredmetal articles prepared: from metal powders and therefore this sinking. effect usually is not encountered after electroplatingv such metal articles with ametallic coating.

Attempts. have been made to overcome" the aforementioned sinking? of the: plated metallic: surface by grinding; or-bufiingf the surface: of. the sintered: meta-l article to; reduce the porosity: of the surface prior to the, electroplating step. This has been successfultoa'. limited degree. However; best resultsare: obtained when grinding: or" buiiing of the sintered; article. is followed by immersion in a; basic: solutionv prior. to the pickling. step;

According'to; my invention,-.this sinking effect of-.- the plated coating on: the sintered metal: ar-

ticle, with. its attendant. disadvantages, is; prevented by' immersing the sintered. metal article in a basicsolutiori' before cleaning or picklin the article in an acid bath. in preparation for theeiectroplatin'g operation Ihave found that by first subjecting the sintered article to the action of a basic solution in. the absence of an electriccurrent,.thepor'es of the sintered article are filled with small amounts oi the basic material thereiby tending-to neutralize any of the acid from the cleani'ngsol'ution or electroplating solution which may laterbecome occluded in the honeycombpassages underneath the electro-deposited metallic coating. Without this prior treatment. in thebasic solution, the trapped acid tends to: attack or dissolve the electroplated metallic coating, causing the" previously described sinking effect and subsequent deterioration of the deposited metallic surface.

The basic solutions employed in practicing my invention may be prepared from materials havinga neutralizing effect on the acids encountered in" the cleaning or picklingsolutions and in the electroplating baths; Among the basic com pounds that may-housed in preparing'the basicsolution are'the'inorganic bases, e; g., ammonium hydroxide, the alkali-metal hydroxides, such as sodium hydroxidapot'assium hydroxide, etcz, alkaline-earth metalihydroxides, e. g., calcium hydroxide, barium hydroxide; eta, the soluble car bonates and bicarbonates of such bases (both hydrous and anhydrous forms), e. g., sodium carbonate, potassium carbonate, calcium bicarbonate, sodium bicarbonate, potassium bicarbonate, etc., and organic bases, for instance trimethyl amine, tributyl amine, dimethyl aniline quinoline, etc. Mixtures of two or more of the aforementioned basic materials also may be employed. I prefer to use sodium carbonate as the basic material in preparing the basic solution.

The basic solution may be in the form of an aqueous solution or other solution medium, as for example, an alcohol solution. Preferably, it is desirable to use an aqueous solution because of the solubility of a greater number of the basic materials in such a medium. In addition, the economy involved recommends the use of water as a solvent. However, due to poor solubility of some of the basic materials in the above-mentioned solvents, it may benecessary .to dissolve the basic material in solvents other than water or alcohol. Among these may be mentioned benzene, toluene, xylene, etc.

The strength or degree of basicity of the basic solution may also be varied. The percentage by baths containing, by weight, from about 1 to 6 or 8 parts of the basic material (e. g., sodium carbonate) per hundred parts of solvent, e. g., water, have been found adequate.

The sintered metal article may be immersed in the basic solution under a variety of temperature and pressure conditions, for instance at normal or at elevated temperatures of the basic solution and at atmospheric or superatmospheric pressures. to boiling or near-boiling temperatures before contacting the sintered metal article with the basic solution. The higher temperatures assist in better impregnating the pores of the sintered article with the basic solution.

The length of time that the sintered metal article may be immersed in the basic solution may vary, 'e. g., from about 1 minute to 2 hours, depending on such influencing factors as, for example, the temperature of the solution, size and composition of the sintered metal article, etc. I have found that when boiling 0r near-boiling basic solutions are employed, the sintered metal article need only be immersed for a short period of time, of the order of about 2 to 10 minutes, or as little as 4 to 6 minutes.

The cleaning or pickling treatment of the sintered metal article, following treatment in the basic solution, may take place. in any of the strong acid solutions and mixtures of acid solutions Well known in the art and used for that purpose. Thus the cleaning or pickling solution may be a 50% aqueous hydrochloric or sulfuric acid bath, or a mixture of aqueous hydrochloric and sulfuric acid solutions. Other acids or mixtures of acids in varying concentrations may be employed in the cleaning solution, depending on the desired strength and acidity of the cleaning solution. Among the acids which maybe mentioned are the inorganic acids including hydrochloric, sulfuric, nitric, phosphoric acids, etc.; or-

I prefer that the basic solution be heated 4 is a sufficient length of time.

ganic acids, e. g., acetic acid (glacial), etc., as well as other types of acids represented by such compounds as ethyl phosphoric acid, ethyl sulfuric acid, etc. The cleaning solution may be heated if desired.

The time of immersion of the sintered metal article in the cleaning solution may be varied depending, for example on the concentration and temperature of the solution, type of acid or acids employed, composition of the sintered metal article, etc. I have found that immersion in cleaning solutions consisting of 50% sulfuric acid or hydrochloric acid, from seconds to 2 minutes The cleaned article may be rinsed in water after removal from the acid cleaning solution.

The electroplating step may consist of any of the methods well known in the art for electrolytically depositing coatings of metals on the surface of metallic articles. Among the coatings which may be deposited electrolytically are chromium, copper, nickel, indium, iron, lead, palladium, tin, zinc, etc., and alloys of two or more of these and other metals.

After removal of the plated sintered metal article from the acid electroplating bath, it is desirable to rinse the article in water and immerse it again for a short time in the same basic solution used prior to the electroplating operation in order to neutralize any acidremaining on the surface of the plated article. This latter step serves to neutralize any acidic material remaining on the surface of the plated article which might interfere with the permanence of the finish. The article is then washed and dried.

It is also desirable, but not necessary, in practicing my invention, to grind or buff the surface of the sintered metal article prior to immersion in the basic solution preceding the acid cleaning and electroplating steps. Surface-grinding assists in decreasing the number and size of the pores on the surface of the sintered metal objects.

In order that those skilled in the art may better understand how the present invention may be practiced, the following example is given by way in a 50% aqueous hydrochloric acid pickling solution. The iron plate treated in this manner was then plated electrolytically at 6 amperes per square inch for 20 minutes in a chromium electroplating bath containing the following ingredients:

33 ounces chromium oxide (CIO3) per gallon of water 0.33 ounce sulfuric acid (specific gravity 1.84) per gallon of water The iron plate was removed from the electroplating bath, rinsed with water, immersed again for about a minute in the 3% aqueous sodium car bonate solution, rinsed and dried. The sample showed no sinking effect of the deposited chromiurn. The surface was shiny, smooth and even. The surface remained in this condition indefinitely without any noticeable change.

When another sampleof the surface-ground, sintered, iron plate was treated exactly as above,

except that the plate was not immersed in the basic solution prior to dipping in the pickling solution, the chromium surface exhibited the previously described pitted, sinking effect. Upon exposure to the air, rusting of the surface soon occurred. This rusting became more pronounced as the time of exposure to the oxidizing effect of the air continued. N o rusting whatsoever was noticed, even after long service use, of the surface of the plated, sintered iron plate which had been immersed in the sodium carbonate solution prior to the electroplating operation.

My method of treating sintered metal articles whereby such articles are subjected to the action of a basic solution prior to the pickling and electroplating operations is not to be confused with methods in existence in the art for cleaning metal parts by treating the parts with a strongly caustic solution, or treating the parts first with a strongly caustic solution and then with a strongly acid solution for the purpose of merely cleaning said metal parts.

The strengths of the basic solutions employed in practicing my invention are usually of such an order that little, if any, cleaning of the sintered metal article is accomplished. The basic solution used in carrying my invention into effect impregnates the pores of the sintered metal article and provides a neutralizing agent for any acid which later may become occluded in its' pores as a result of cleaning the article in the acid cleaning or pickling solution. Its function is also to neutralize any acid which may become trapped in the pores of the sintered article as a result of the electroplating operation in the acid electroplating solution.

My invention is applicable to sintered metal objects manufactured from metallic powders which have been compacted, preformed and molded under high pressures, and then sintered at elevated temperatures to cause flow and cohesion of the particles of metal powder whereby strong, homogeneous articles are formed. Articles prepared in this manner exhibit the type of surface porosity which heretofore has proved difficult to electroplate properly with another metallic coating. However, my invention may be applied to metal objects wherein the porosity of the metal base prevents the deposition of smooth coatings of metallic materials by the usual electrolytic plating processes. Articles prepared by casting of molten metals or by machining of unsintered metal blanks do not usually present the problem of porosity found in the case of sintered metal articles.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. The process for electroplating a sintered ferrous article containing a multitude of microscopic pores on its surface thereby to obtain a metallic coating free of a pitted, sinking effect, which process consists essentially of (1) surface-grinding the sintered ferrous article, (2) immersing the said sintered article in the absence of an electric current so as to impregnate it in a heated 0.5 to 30 per cent aqueous alkaline solution maintained at around its boiling point thereby to 00- clude small amounts of the alkaline solution within the pores of the sintered article, (3) removing the article and washing it with water, (4) immersing the washed article in and subjecting it to the action of an acid cleaning solution, (5) removing the article from the acid cleaning solution and electrolytically depositing a chromium coating on the surface of the cleaned article using an acid electroplating bath, (6) removing the electroplated article and rinsing it with water, (7) immersing the article again in the aforementioned alkaline solution to neutralize any acid remaining on the surface of the plated article, and (8) rinsing the article with water and drying it.

2. The process as in claim 1 wherein the aqueous alkaline solution employed in step 2 for impregnating the pores of the sintered article comprises, by weight, from about 1 to 8 parts of the alkaline material per parts of water solvent.

3. The process as in claim 1 wherein the aqueous alkaline solution comprises an aqueous sodium carbonate solution containing, by weight, from about 1 to 8 parts of sodium carbonate per 100 parts of water solvent.

RANDALL B. TINKER.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,878,837 Francis Sept. 20, 1932 2,028,312 Bornhauser Jan. 12, 1936 2,323,162 Talmadge June 29, 1943 OTHER REFERENCES Chemical and Metallurgical Engineering, Dec. 1942, page 166-12.

Special Volume on Modern Electroplating, published by Electrochemical Society, 1942; pages 120, 131, 132, 134.

Metal Industry, Electrometallurgy Supplement, May 12, 1944, pages 299, 300. 

